Ignore desiredCount of ECS when it's 0 or not set for AutoScaling

docs/content/en/docs-dev/user-guide/configuration-reference.md

@@ -516,6 +516,8 @@ One of `yamlField` or `regex` is required.
 
 There are some restrictions in configuring a service definition file.
 
+- As long as `desiredCount` is 0 or not set, `desiredCount` of your service will NOT be updated in deployments.
+  - If `desiredCount` is 0 or not set for a new service, the service's `desiredCount` will be 0.
 - `capacityProviderStrategy` is not supported.
 - `clientToken` is not supported.
 - `deploymentController` is required and must be `EXTERNAL`.

docs/content/en/docs-dev/user-guide/managing-application/defining-app-configuration/ecs.md

@@ -155,6 +155,7 @@ spec:
   - That means you need to link target groups to your listener rules before deployments.
   - For more information and diagrams, see [Issue#4733 [ECS] Modify ELB listener rules other than defaults without adding config](https://github.com/pipe-cd/pipecd/pull/4733).
 - When you use [Service Connect](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service-connect.html), you cannot use Canary or Blue/Green deployment yet because Service Connect does not support the external deployment yet.
+- When you use auto scaling for a service, you can avoid reconciling `desiredCount` by not setting `desiredCount` in the service definition. See [Restrictions of Service Definition](../../../configuration-reference/#restrictions-of-service-definition).
 
 ## Reference
 

pkg/app/piped/executor/ecs/ecs.go

@@ -278,8 +278,7 @@ func sync(ctx context.Context, in *executor.Input, platformProviderName string,
 		cnt := service.DesiredCount
 		// Scale down the service tasks by set it to 0
 		in.LogPersister.Infof("Scale down ECS desired tasks count to 0")
-		service.DesiredCount = 0
-		if _, err = client.UpdateService(ctx, *service); err != nil {
+		if err = client.PruneServiceTasks(ctx, *service); err != nil {
 			in.LogPersister.Errorf("Failed to stop service tasks: %v", err)
 			return false
 		}

pkg/app/piped/platformprovider/ecs/client.go

@@ -120,16 +120,36 @@ func (c *client) CreateService(ctx context.Context, service types.Service) (*typ
 	return output.Service, nil
 }
 
+func (c *client) PruneServiceTasks(ctx context.Context, service types.Service) error {
+	input := &ecs.UpdateServiceInput{
+		Cluster:      service.ClusterArn,
+		Service:      service.ServiceName,
+		DesiredCount: aws.Int32(0),
+	}
+
+	_, err := c.ecsClient.UpdateService(ctx, input)
+	if err != nil {
+		return fmt.Errorf("failed to update ECS service %s: %w", *service.ServiceName, err)
+	}
+
+	return nil
+}
+
 func (c *client) UpdateService(ctx context.Context, service types.Service) (*types.Service, error) {
 	input := &ecs.UpdateServiceInput{
 		Cluster:              service.ClusterArn,
 		Service:              service.ServiceName,
-		DesiredCount:         aws.Int32(service.DesiredCount),
 		EnableExecuteCommand: aws.Bool(service.EnableExecuteCommand),
 		PlacementStrategy:    service.PlacementStrategy,
 		// TODO: Support update other properties of service.
 		// PlacementConstraints:    service.PlacementConstraints,
 	}
+
+	// If desiredCount is 0 or not set, keep current desiredCount because a user might use AutoScaling.
+	if service.DesiredCount != 0 {
+		input.DesiredCount = aws.Int32(service.DesiredCount)
+	}
+
 	output, err := c.ecsClient.UpdateService(ctx, input)
 	if err != nil {
 		return nil, fmt.Errorf("failed to update ECS service %s: %w", *service.ServiceName, err)

pkg/app/piped/platformprovider/ecs/ecs.go

@@ -45,6 +45,7 @@ type ECS interface {
 	ServiceExists(ctx context.Context, clusterName string, servicesName string) (bool, error)
 	CreateService(ctx context.Context, service types.Service) (*types.Service, error)
 	UpdateService(ctx context.Context, service types.Service) (*types.Service, error)
+	PruneServiceTasks(ctx context.Context, service types.Service) error
 	WaitServiceStable(ctx context.Context, service types.Service) error
 	RegisterTaskDefinition(ctx context.Context, taskDefinition types.TaskDefinition) (*types.TaskDefinition, error)
 	RunTask(ctx context.Context, taskDefinition types.TaskDefinition, clusterArn string, launchType string, awsVpcConfiguration *config.ECSVpcConfiguration, tags []types.Tag) error